Method of making average mass flow velocity measurements employing a heated extended resistance temperature sensor

ABSTRACT

An RTD (resistance temperature sensor or detector) sensing device which is a long, thin, unitary device adapted to be distributed across an extended field for the continuous, uninterrupted sensing or interrogation of such field, avoiding the inaccuracy, unreliability, and excessive expense of conventional &#34;point&#34; RTD and thermocouple sensors currently employed for this purpose. According to the invention, a very long, thin, ductile protective metal outer sheath houses a coextensive body of insulation material, which in turn supports and electrically insulates one or more coextensive RTD filaments and in most forms of the invention one or more heater filaments. Distributed RTDs of the invention may, along their lengths, have continuous linear function sensitivity, continuous variable function sensitivity, or step function sensitivity. Distributed RTDs of the invention have particular utility for gauging liquid level, measuring average mass flow velocity of fluids in large ducts, and sensing the average temperature of an extended nonisothermal field.

This is a continuation of application Ser. No. 07/932,233 filed on Aug.19, 1992, now U.S. Pat. No. 5,355,727 and which is a division of Ser.No. 07/543,337, filed on Jun. 25, 1990, now U.S. Pat. No. 5,167,153.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electrical resistance temperaturesensors or detectors (RTDs), and it relates more particularly to a long,slender, continuous RTD capable of sensing continuously over an extendedfield.

2. Description of the Prior Art

Both thermocouples and resistance temperature sensors (RTDs) are inwidespread use for sensing temperature and providing an electricaloutput representative of the temperature sensed. Thermocouples, by theirnature, are point sensors because they thermoelectrically produce ane.m.f. at a specific junction between two different metals. RTDs employa wire sensing element which has a resistance that varies according totemperature. Present RTDs are designed to concentrate the electricalresistance at a small point or in the smallest possible volume, withminiaturization being a principal feature so that RTDs are, likethermocouples, essentially point sensors. Because of this point sensingfeature of both thermocouples and RTDs, wherever an extended field is tobe interrogated with the use of either thermocouples or RTDs, it hasheretofore been necessary to distribute a multiplicity of thermocouplesor RTDs at selected points in the field. No matter how many pointsensing thermocouples or RTDs are distributed in a field, they still areunable to provide an accurate analog representation of the informationto be determined from the field, because they are still only sensingspecific points in the field. Determining the best points tointerrogate, installing the individual thermocouples or RTDs, and makingthe numerous required individual electrical connections to the pointsensing thermocouples and RTDs as is currently done is cumbersome andexpensive.

One type of sensing of an extended field by the point sensingthermocouples and RTDs is sensing of the average temperature of a field.It will be apparent that the larger such a thermal field is, and themore varied the temperatures across the thermal field, the more pointsensing thermocouples or RTDs are required to obtain an average readoutwhich is fairly representative of the average temperature of the field.

Another type of extended field interrogation currently made withthermocouples or RTDs involves gauging of the level or location of aphase change interface, such as the liquid level, or interface betweenliquid and gas, in a tank. Such liquid level gauging is currentlyaccomplished with thermocouples and RTDs by arranging a series of spacedthermocouples or RTDs along the height of the tank, i.e., at verticallyseparated points in the field being interrogated. Where RTDs areemployed for this purpose, a series of heated RTDs and companionreference RTDs are employed along the height of the tank. As liquidreaches each RTD point sensor, the sensor reports that it is wet when itbecomes cooled by the higher thermal dispersion rate of the liquid thanthe air above it. However, the operator is unable with such pointsensing to determine whether the liquid level is just at that particularpoint or at any level between that point and just below the next higherRTD sensing point. Further filling of the tank will result in discretereports from the sequentially higher RTDs, while lowering of the liquidlevel in the tank will cause successive discrete reports fromsuccessively lower RTDs as they are uncovered from the liquid. Forexample, if ten sensing points are employed along the height of thetank, each with an individual heated RTD sensor and a reference RTDsensor, the gauging can only be performed at ten individual steppedpoints, with total uncertainty of where the liquid level is between thepoints. The only way to reduce such uncertainty is to increase thenumber of sensing points, at correspondingly increased expense. A liquidlevel sensing system of this type is disclosed in applicant's U.S. Pat.No. 4,449,403, issued May 22, 1984 for "Guide Tube Inserted Liquid LevelSensor."

Accurate liquid level sensing is of critical importance in liquidstorage vessels and reactor buildings and in the reactor vesselsthemselves of nuclear power plants to avoid accidents such as that atthe Three Mile Island plant, where liquid level was misinterpreted.Where a series of vertically arrayed point sensing thermocouples or RTDsis employed to determine liquid level, not only is there a lack ofdesired accuracy by not knowing where the liquid level is between thesensing points, but liquid level changes may not be immediately sensed,since there can be a considerable change in liquid level prior todetection, so that a developing problem may not be immediately detected,and therefore mitigating action to suppress the problem would not bepromptly taken by the operator.

Since each of the vertical sequence of thermocouples or RTDs in suchpresent liquid level gauging systems requires its own separateelectrical connections to the detection circuitry, the required largenumber of electrical joints or splices results in undesirably lowreliability, which could be dangerous in the nuclear power plantenvironment. As an illustration of how serious this problem can be,applicant is familiar with one point-sensing RTD system for gaugingwater level in a nuclear reactor building which has as many as fifty RTDsensors arrayed over a vertical height of approximately sixty feet.

RTDs are generally preferred for some purposes over thermocouples formost uses because they can be made much more sensitive, being able toprovide an output signal many times greater than thermocouples. This isbecause RTDs operate with an external electrical power source, which canprovide as high a voltage or current as is desired, whereasthermocouples operate on the basis of a self-generated junction e.m.f.,which inherently has a very low output voltage level as well as otherinaccuracies. Nevertheless, for sensing some extended fields, such asthe inside of a nuclear reactor vessel, access may be difficult, andbest achieved by encasing a series of the sensors in a long, slendertubular probe. Such a probe can readily be inserted in an existingreactor vessel instrument guide tube. While it would be desirable tohave RTDs so packaged because of their high output, and hencesensitivity, current state of the art RTDs are not suitable for suchpackaging, being much too bulky, and having a ceramic or glass insulatortoo brittle to allow them to be deformed as would be required forpackaging them in such a long, slender tubular probe. Thermocouples, onthe other hand, have been known to be packaged inside a metal casing assmall as 0.010 inch in diameter, and a series of such encasedthermocouples and the required electrical leads placed inside a tube andencased by drawing or swaging the tube down around the thermocouples andleads to produce a long, slender probe suitable for gaining access toconstricted regions inside a nuclear reactor vessel. However, suchthermocouple probes have serious disadvantages. First, the thermocouplesare delicate and are easily subject to breakage during the manufactureof such probes or upon accidental impacting. Also, because of theirinherent point sensing, the thermocouple-type probes necessarily have astep function output, rather than a continuous output, so liquid levelcannot be accurately determined. Further, the electrical output of thethermocouples is so small that performance is grainy and resolution andaccuracy are poor. Also, individual wire leads are required for each ofthe thermocouples, so that numerous wires must extend along the tube ofthe probe, which seriously limits how small the outside diameter of thetubular probe can be, and of course the larger the number ofthermocouples placed along the probe in an attempt to increaseresolution, the greater the number of leads. The large number of leadsalso seriously reduces the reliability of such thermocouple-type probes.Such thermocouple-type probes are also quite expensive to make, and itis even more expensive to provide leads, connections and electroniccooperating devices for thermocouple-type probes.

Another type of extended field which has been interrogated by amultiplicity of RTDs or thermocouples is a large duct having anonuniform flow profile, where it is sought to obtain an average readingof the flow velocity in the duct. Such nonuniform flow distributionsexist, for example, in air ducts where diameters are large and fittingssuch as tees, elbows, transitions, bends, section changes, louvers,dampers, and the like cause flow disturbances. Nonuniform flowdistributions also typically occur in the input air ducts and combustionoutput ducts of fossil fuel power plants. In such cases, a multiplicityof point sensing RTD or thermocouple sensors are placed at what areconsidered to be strategic locations across the air or gas flow path,but only a rough approximation of the flow velocity can be obtained byuse of such discrete, point sensing locations. Again, these individualRTD point sensors suffer from high costs of leads, connectors, andmating electronic devices that cooperate in interpreting the individualsignals.

SUMMARY OF THE INVENTION

In view of these and other problems in the art, it is a general objectof the present invention to provide an RTD of very long, thinconfiguration suitable for interrogating an extended field.

Another general object of the invention is to provide a long, thin RTDwhich, disposed along or across an extended field, is capable of servingthe same sensing function as a multiplicity of point sensing-type RTDsor thermocouples distributed over the field, and which therefore may beconsidered as a unitary distributed RTD.

Another object of the invention is to provide a long, thin distributedRTD which is capable of providing infinitessimally continuous,uninterrupted sensing, as compared to the discontinuous, interrupted,step function-type sensing of conventional point sensing RTDs andthermocouples.

Another object of the invention is to provide a distributed RTD whichhas a high degree of sensitivity and has an output with excellentresolution.

Another object of the invention is to provide a long, thin distributedRTD which may be made as long as desired for spanning any particularfield to be sensed.

Another object of the invention is to provide a long, thin distributedRTD of the character described which is flexible such that it can berolled up for convenience of storage, shipping, and installation incramped quarters.

A further object of the invention is to provide a distributed RTDcapable of providing an accurate analog representation of informationsensed in an extended field, as compared to an averaging of specificfinite points in the field or a step function output of the level orlocation of a phase change interface such as the liquid level in a tank.

A further object of the invention is to provide a long, slenderdistributed RTD which is capable of accurately sensing the averagetemperature of an extended field.

A further object of the invention is to provide a distributed RTD of thecharacter described, which, when deployed as a matched pair along thevertical height of a tank, is capable of accurately gauging the liquidlevel in the tank on a continuous, uninterrupted basis.

A still further object of the invention is to provide a distributed RTDof the character described which is capable, deployed as a matched pair,of measuring the average mass flow velocity of gas flow in an irregularregion of a large duct where there is nonuniform flow velocitydistribution.

Yet another object of the invention is to provide, for the first time,RTD packaging which is long, thin, and continuous, and which can be madeeven thinner than multiple channel thermocouple probes for sensing infields normally difficult to access such as inside a nuclear reactorvessel, yet which has the high sensitivity and resolution capability ofRTD sensors.

Another object of the invention is to provide a distributed RTD sensorcapable of sensing over an extended field, yet which has a high degreeof reliability because it requires only a minimum number of electricalconnections, and because its operative filaments are well protected in astrong outer metal casing or sheath. The outer casing may be providedwith spring-like flexibility and toughness which avoids likelihood ofimpact damage to the RTD and heater filaments or of sharp bends or kinksbeing formed therein from handling, and enables the long, thindistributed RTD to be rolled up for convenience of storage, shipping,and installation in cramped quarters.

A further object of the invention is to provide a distributed RTD of thecharacter described which is relatively inexpensive to manufacture, andwhich is particularly inexpensive to install because it does not requirethe numerous attachments and many electrical connections and junctionsassociated with point sensing thermocouples and RTDs distributed aboutan extended field.

A still further object of the invention is to provide a distributed RTDof the character described wherein RTD filament material may benonlinearly arranged along its length so as to accommodate or compensatefor a nonlinear field.

A still further object of the invention is to provide a long, thindistributed RTD of the character described wherein portions thereof maybe so arranged as to provide a step function output.

Another object of the invention is to provide a distributed RTD sensorof the character described in which the conductors connecting the RTDand heater filaments of the sensor to remote detection circuitry arehoused in the same continuous outer sheath as the sensor filaments toavoid any junctions near the region being sensed.

Yet another object of the invention is to provide a method of increasingsensitivity of distributed RTD devices of the invention to mass fluidflow which involves utilizing relatively larger amounts of the RTDmaterial in regions of relatively slower fluid flow.

An additional object of the invention is to provide configurations andmethods for correlating equal incremental lengths of the distributedRTDs of the invention with equal areas in ducts to provide a signaloutput truly representative of average mass flow rates through theducts.

According to the invention, an RTD sensing device is provided in a long,thin, linear configuration capable of spanning and interrogating anextended field. The basic structure of the invention in its simplestform consists of an elongated, thin, tubular outer sheath preferablymade of a ductile metal such as stainless steel or other suitable metal,or made of other tough material such as plastic, that is capable ofbeing drawn out from a relatively short, thick starting stage to thefinal long, thin configuration. An elongated body of electricalinsulation material, preferably mineral insulation material such asalumina or magnesia, is contained within the outer metal sheath andextends longitudinally generally coextensively with the sheath. At leastone long, thin filament of RTD material is supported within theinsulation body and extends longitudinally generally coextensively withthe insulation body and the sheath, the RTD filament along its lengthbeing transversely physically separated from and electrically insulatedfrom the sheath by the insulation body. Electrical connections are madeto end portions of the RTD filament for connection to detectioncircuitry, which may be of either the constant voltage type or theconstant current type. A simplified form of RTD according to theinvention does not include a heater filament as a companion to the RTDfilament, add this form has utility as a linear thermometer adapted tohave its length disposed across or along an extended nonisothermaltemperature field for sensing the average temperature of the field, ascompared to the conventional costly deployment of a multiplicity ofpoint sensing RTDs or thermocouples.

A distributed RTD of the invention which does not have an internalheater filament may be made as a heated distributed RTD by making theouter sheath out of a high resistance ductile metal, and electricallyenergizing the sheath so that it will serve as a heater.

The other forms of the invention include one or more electricallyenergizable heater filaments generally coextensive with one or more RTDfilaments, the heater filament or filaments being supported in theinsulation body so as to be thermally coupled with the RTD filament orfilaments but physically separated and electrically insulated from theRTD filament or filaments. The heater-type distributed RTDs of theinvention are particularly useful for gauging liquid level on acontinuous, linear, nonstepped basis, and for measuring the average massflow velocity of gas flowing in an irregular region of a large ductwhere the irregularity causes nonuniform flow velocity distribution. Forsuch uses of the heater-type distributed RTDs of the invention, they arepreferably deployed in matched parallel pairs, with the heater filamentor filaments of one of a matched pair being energized, while the heaterfilament or filaments of the other is left unenergized, the unheateddistributed RTD serving as a thermal reference.

In preferred forms of the invention, a plurality of RTD filaments eachextend the length of the distributed RTD, and are electrically connectedpreferably in series so as to multiply the sensitivity and resolution ofthe distributed RTD. By utilizing an even number of suchseries-connected filaments, both of the outside electrical connectionsare enabled to be made at one end of the distributed RTD, which makesconnection to detection circuitry much more convenient than ifconnections must be made from both ends of the very long distributedRTD.

All of the long, thin, linear forms of the invention are made byinitially providing the outer sheath, insulation body, and RTD andheater filaments in relatively short, thick form, assembling them insuch form, and then swaging and/or drawing the assembly as a unit in aseries of passes out to its final very long, thin configuration, andduring such swaging and/or drawing the parts remain in their samerelative proportions and locations.

In some forms of the invention, the effective length of RTD filament isgreatly increased for increase of sensitivity and resolution byarranging the RTD filaments in a long, thin, helical array. This isaccomplished by employing one or more extremely long linear forms of theinvention helically coiled along a thin tubular mandrel which preferablyhouses the heater filaments.

For most purposes, it is desired to have a sensitivity of thedistributed RTD which is linear along its length. However, the helicalform of distributed RTD may have its coils wound nonlinearly so as toprovide any desired nonlinear sensitivity along the length of thehelical distributed RTD. Similarly, a step function may be providedalong the length of the helical-type distributed RTD of the invention byhaving a series of helical clusters regularly or irregularly spacedalong the length of the helical RTD, the clusters providing highsensitivity steps, with the clusters being interconnected by straight,axial RTD sections of relatively low sensitivity.

The step function may alternatively be accomplished in distributed RTDsof the invention by employing one or more heater filaments that aresegmented into alternate high resistance and low resistance sections,the high resistance sections providing high sensitivity for thecoextensive sections of RTD filament, and the low resistance sectionsproviding regions of low sensitivity for the coextensive sections of RTDfilament. A similar step function can be provided by employing one ormore RTD filaments that are segmented into alternate RTD and non-RTDsections, or alternate sections of high and low RTD sensitivity. A stepfunction can also be accomplished by employing alternating sections ofinsulation material having high and low thermal conductivity.

A form of the invention which is particularly useful in atomic powerplants has the conductors connecting the RTD and heater filaments of thesensor to remote detection circuitry housed in the same continuous outersheath as the sensor filaments to avoid junction boxes or otherconnecting devices in sensitive areas.

All forms of the present invention may be made very long and very thin.Thus, linear or nonhelical forms of the invention can be made with anoutside diameter as small as from approximately 0.010 inch toapproximately 0.030 inch, and can be made as long as several hundredfeet if desired. A helical form of the invention with a single helixlayer can be made with an outside diameter as small as approximately 1/8inch, and in lengths of sixty feet or more if desired.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects of the invention will become more apparent whentaken in conjunction with the drawings, wherein:

FIG. 1 is a diametrically enlarged, fragmentary perspective view of aform of the invention which has a single RTD filament and does notinclude a heater filament;

FIG. 2 is a greatly enlarged transverse sectional view taken on the line2--2 in FIG. 1;

FIG. 3 is a fragmentary axial section, partly in elevation, taken online 3--3 and the scale of FIG. 2, showing the distributed RTD of FIGS.1 and 2;

FIG. 4 is a billet-like starting assembly which is to be swaged and/ordrawn to produce the distributed RTD of FIGS. 1-3;

FIG. 5 is a view similar to FIG. 1, but illustrating a distributed RTDof the invention having both an RTD filament and a heater filament;

FIG. 6 is a greatly enlarged transverse sectional view taken on the line6--6 in FIG. 5;

FIG. 7 is a fragmentary axial sectional view, partly in elevation, takenon the line 7--7 in FIG. 6 and having the same scale as FIG. 6;

FIG. 8 is a greatly enlarged, fragmentary axial sectional view, partlyin elevation, illustrating another form of the invention which has apair of RTD filaments electrically connected in series, and a singleheater filament;

FIG. 9 is a transverse sectional view taken on the line 9--9 in FIG. 8and having the same scale as FIG. 8;

FIG. 10 is a greatly enlarged, fragmentary perspective view which isaxially sectional with portions in elevation, illustrating a furtherform of the invention which has four RTD filaments electricallyconnected in series, and a single heater filament;

FIG. 11 is a view similar to FIG. 10, but illustrating another form ofthe invention which has a pair of RTD filaments electrically connectedin series, and a pair of heater filaments electrically connected inseries;

FIG. 12 is a greatly enlarged, fragmentary axial sectional view, withportions in elevation, illustrating a helical form of the inventionhaving a linear distributed RTD of the invention helically coiled in asingle layer, and with phantom lines indicating the great length of thehelically coiled distributed RTD relative to its diameter;

FIG. 13 is a still further enlarged, fragmentary axial sectional view,partly in elevation, taken on the line 13--13 in FIG. 12, showing theclosed end portion of the linear distributed RTD which is helicallycoiled in the helical distributed RTD of FIG. 12;

FIG. 14 is a greatly enlarged, fragmentary axial section, partly inelevation, taken on the same scale as FIG. 12, showing another helicalform of the invention in which the helically coiled linear RTD elementis encased in a potting or other filler material for protection of thecoil;

FIG. 15 is a greatly enlarged, fragmentary axial section, partly inelevation, similar to FIG. 12 but on a somewhat smaller scale,illustrating a double helical form of the invention having two linearRTDs helically coiled in two layers along the length of the helical RTD;

FIG. 16 is a fragmentary vertical section, partly in elevation,illustrating a tank with a matched, parallel pair of distributed RTDs ofthe invention vertically deployed along a wall of the tank for liquidlevel gauging;

FIG. 17 is a side elevational view, with a portion broken away,illustrating a duct elbow with a matched pair of distributed RTDs of theinvention deployed across it;

FIG. 18 is a transverse sectional view, with portions shown inelevation, taken on the line 18--18 in FIG. 17, with the diameters ofthe distributed RTDs exaggerated relative to their lengths forillustrative purposes;

FIG. 19 is a transverse vertical section, partly in elevation,illustrating a right circular cylindrical tank laid on its side, with amatched pair of the helically coiled form of distributed RTDs of theinvention vertically deployed in the tank, the pair of distributed RTDsbeing diagramatically illustrated as having the coils of the helicalwinding variably separated along their lengths with the windingvariations characterized to represent the curvature of the tank so as toprovide a linear output representing liquid quantity in the tank;

FIG. 20 is a fragmentary vertical sectional view, partly in elevation,illustrating a tank with a vertically deployed matched pair ofdiagrammatically illustrated distributed RTDs of the invention havingalternating helical and straight RTD sections for providing a stepfunction output with linear output between the steps;

FIG. 21 is a greatly enlarged, fragmentary axial section, partly inelevation, illustrating another step function form of the invention inwhich the heater filament has alternating high resistance and lowresistance sections;

FIG. 22 is a view similar to FIG. 21, illustrating a still further stepfunction form of the invention in which the insulation body hasalternating sections of relatively high thermal conductivity andrelatively low thermal conductivity;

FIG. 23 is a block diagram illustrating a constant current circuit whichmay be utilized in connection with any of the forms of the presentinvention;

FIG. 24 is a view similar to FIG. 16, but with the distributed RTDsdisposed within a still well to mitigate the effects of fluidturbulence;

FIG. 25 is an enlarged, fragmentary, horizontal sectional view taken onthe line 25--25 in FIG. 24;

FIG. 26 is a view similar to FIG. 18, but with the distributed RTDsdisposed within a perforated shroud to prevent signal saturation by highfluid flow velocities through the duct;

FIG. 27 is an enlarged, fragmentary sectional view taken on the line27--27 in FIG. 26;

FIG. 28 is a diagrammatic cross-sectional view of a duct illustratinglinear RTD means of the invention diametrically deployed across the ductand variably sinuously configured to provide a variable functionresponse corresponding to the variable diametrical area function offluid flow symmetrical about its diametrical axis;

FIG. 29 is a diagrammatic cross-sectional view of a duct illustratinglinear RTD means of the invention arrayed in a spiral configuration toprovide a variable function response corresponding to the variableradial area function of fluid flow symmetrical about the center axis ofthe duct; and

FIG. 30 is a greatly enlarged fragmentary axial sectional view, withportions in elevation and portions broken away, of a form of theinvention wherein a continous outer sheath encloses and protects boththe distributed RTD and heater filaments of the invention and theelectrical conductors connecting such filaments to detection circuitry.

DETAILED DESCRIPTION

FIGS. 1-3 illustrate a temperature sensing form of the present inventionwhich embodies a long, slender RTD sensing element encased in an innercylindrical electrical insulation body, preferably of mineral insulationmaterial, which, in turn, is encased in a cylindrical outer sheath whichis preferably made of metal, but alternatively may be made of othertough material such as a suitable plastic. This form of the inventiondoes not include the long, slender heater element which is a companionto the RTD element in other forms of the invention. The form of theinvention illustrated in FIGS. 1-3 is a distributed RTD temperaturesensor adapted to have its length disposed across or along an extendedfield for continuous interrogation or sensing of the average temperatureof the field, as compared to the costly conventional procedure ofemploying a multiplicity of "point" sensing thermocouples or RTDs withtheir numerous required individual electrical connections.

The distributed RTD of the invention, including the simplified formillustrated in FIGS. 1-3 and the other forms of the invention disclosedin the other figures of the drawings and described hereinafter indetail, has a very long, very thin configuration, which is a primarynovel feature of the present invention in the RTD art, which hasheretofore been directed principally toward miniaturization of RTDs tocome as close as possible to "point" sensing. Distributed RTDs accordingto the invention may be made as long as required for spanning anyparticular field or zone, and if desired can be as long as severalhundred feet. The cross-sectional dimension of distributed RTDsaccording to the invention, on the other hand, may be extremely small,as for example having an external diameter in the range of fromapproximately 0.010 inch to approximately 0.030 inch. Such long, thinconfiguration of RTDs according to the invention is enabled because thelong, thin RTD sensor filaments of the invention are encased along theirentire lengths first in a structurally stable mineral insulation body,and then in a tough, cylindrical outer sheath. Distributed RTDsaccording to the invention will generally be provided with a length thatis greater than approximately 100 times its diameter, and in mostinstances it will be provided with a length that is hundreds, or eventhousands, of times greater than its diameter.

The distributed RTD illustrated in FIGS. 1-3 is generally designated 10,and has a cylindrical outer sheath 12 with ends 14 and 16 whichconstitute the ends of distributed RTD 10. Outer sheath 12 constitutesthe primary structural body of distributed RTD 10. Sheath 12 ispreferably made of metal, preferably a stainless steel such as 316stainless, which has a high degree of ductility, a requirement forenabling the sheath 12 to be swaged and/or drawn down from a muchshorter, larger diameter cylindrical starting body. Sheath 12 alsopreferably has a high degree of corrosion and/or abrasion resistance,which is desirable because of the likelihood of the distributed RTDbeing subjected to an environment which is hostile chemically and/ormechanically. Alternatively, sheath 12 may be coated with other materialto improve its corrosion and/or abrasion resistance. The swaging ordrawing, or both, of sheath 12 to produce the very long, thinconfiguration of distributed RTD 10 preferably causes sheath 12, if madeof metal, to be work-hardened during the last swaging and/or drawingpass or passes to a final condition of high strength and good springcharacteristics. These physical characteristics of sheath 12, coupledwith the internal compression provided by the mineral insulation bodywithin sheath 12, give the overall assembly of distributed RTD 10 goodstructural strength, spring characteristics, and flexibility which avoidthe likelihood of sharp bends or kinks being formed therein fromhandling, and enable the very long, thin distributed RTD 10 to be rolledup for convenience of storage, shipping, and installation in crampedquarters. Alternatively, the entire assembly of distributed RTD 10 canbe annealed if desired.

While ductile metal is the presently preferred material for sheath 12,it is to be understood that the invention is not limited to the use ofmetal for sheath 12, and sheath 12 may alternatively be made of othertough, ductile material such as a suitable plastic capable of beingswaged and/or drawn out to the final long, thin configuration from ashorter, larger diameter starting body.

Sheath 12 is filled, except for the presence of the RTD filament, alongits entire length between ends 14 and 16 with an uninterrupted body 18of electrical insulation material, which is preferably mineral. This ispreferably alumina (Al₂ O₃) or magnesia (MgO), which both have thefavorable physical characteristics of high electrical resistance, goodthermal conductivity, good compressional structural strength, goodstructural stability, and good thermal stability. The mineral materialof insulation body 18 is composed of powdered mineral material which isobtainable compressed together with a binder as a continuous rigid blockresembling a piece of chalk. Such mineral block material is referred toin the trade as "crushable mineral insulate," and is obtainable fromsuch sources as Norton, Coors, and American Lava. It has thecharacteristic of being crushable and flowable so as to formcoextensively with sheath 12 as sheath 12 is being swaged and/or drawnfrom an initial relatively short, thick configuration out to its verylong, very thin final configuration.

An RTD wire filament 20 extends through the entire length of sheath 12and insulation body 18, being coaxially centrally located withininsulation body 18 so as to be electrically insulated from sheath 12along its length. The RTD material of wire filament 20 is a materialhaving a coefficient of electrical resistance that varies withvariations in temperature, normally increasing for higher temperature.Platinum is the presently preferred RTD material, but other materialssuch as "Alumel," an alloy of aluminum and nickel, "Chromel," an alloyof chromium and nickel, "Balco," an alloy of iron and nickel, or copper,may be used. Each of these RTD materials has the required ductilityenabling it to be swaged and/or drawn to the long, slender configurationof the finished distributed RTD 10. RTD filament 20 is preferably roundin cross-section, as are outer sheath 12 and insulation body 18, andstarts off as a relatively short cylindrical rod which is swaged and/ordrawn down to a very long, thin wire filament concurrently with theswaging and/or drawing of sheath 12 and insulation body 18 so as to becoextensive in length with sheath 12 and insulation body 18.

Preferably, the ends of the distributed RTD 10 are dressed to asquared-off shape upon completion of the swaging and/or drawingoperations, and RTD lead wires 22 and 24 are mechanically andelectrically connected to the respective ends of the drawn RTD wire 20.Such connection is preferably a welded connection for strength and sureelectrical continuity, and lead wires 22 and 24 may either bebutt-welded to the respective ends of RTD filament 20, or some of theinsulation material may be scooped out of the ends 14 and 16 of sheath12 and the welds made within the ends 14 and 16 of sheath 12, insulationmaterial then being back-filled into the end portions of sheath 12 sothat the body 18 of insulation material remains uninterrupted. Weldingcurrent may be applied through the RTD wire filament 20 and the leadwires 22 and 24.

After lead wires 22 and 24 are thus applied, the ends of distributed RTD10 are preferably covered with hermetic seals 25 which serve as moisturebarriers.

Distributed RTD 10 is so long and has such an extremely smallcross-section that it would be difficult to preform each of the parts12, 18 and 20 to their final lengths and cross-sectional dimensions andthen assemble them. Because of this, they are prefabricated andassembled in a billet-like starting assembly 26 such as that illustratedin FIG. 4, and then starting assembly 26 is swaged and/or drawn out tothe final long, slender dimensions. Starting assembly 26 is provided ina length and of a diameter for convenience of assembly and withsufficient of the materials to produce the desired final length anddiameter of distributed RTD 10. The initial length of starting assembly26 may be on the order of 2-3 feet if desired, but this is a matter ofchoice. Starting assembly 26 includes cylindrical outer sheath or casing12a, preferably of metal, having ends 14a and 16a, coaxial cylindricalinsulation block 18a, preferably of mineral insulation material, andcoaxial RTD rod 20a, the casing 12a, insulation block 18a, and RTD rod20a preferably being coextensive in length, and being provided in thesame relative diametral proportions as they are desired to be in thecompleted distributed RTD 10.

Insulation block 18a is externally preformed or machined to acylindrical shape that will slideably fit into cylindrical casing 12a,and an axial hole may be preformed through the length of insulationblock 18a to receive RTD rod 20a.

Conventional wire and tube swaging and drawing techniques may beemployed, and if desired, either swaging, or drawing, or a combinationof swaging and drawing techniques may be applied. The first drawing orswaging pass over the outside of sheath or casing 12a will tightly clampcasing 12a against insulation block 18a and tightly clamp RTD rod 20awithin insulation block 18a along the length of starting assembly 26,with a close molecular fitting together of the parts such that the partswill be lengthened coextensively during all succeeding swaging and/ordrawing passes until the desired final very long, very thinconfiguration of distributed RTD 10 is achieved. Annealing passes willnormally be made between most of the swaging and/or drawing passes tokeep casing 12a ductile for further forming. Such coextensivelengthening of the starting casing 12a, insulation block 18a, and RTDrod 20a will result in the same cross-sectional proportions of finalsheath 12, insulation body 18, and RTD filament 20 in distributed RTD 10as the starting cross-sectional proportions of casing 12a, insulationblock 18 a, and RTD rod 20a in the starting assembly 26, and RTDfilament 20 remains centered in insulation body 18.

As indicated above for the distributed RTDs of the invention in general,the finished distributed RTD 10 shown in FIGS. 1-3 and formed from thebillet-like starting assembly 26 shown in FIG. 4, may have an OD(outside diameter) as small as on the order of 0.010 to 0.030 inch, anda length as long as required to span a particular field or zone to besensed, with lengths of several hundred feet being practical for theinvention. The diameter for the completely swaged and/or drawn RTD wirefilament 20 of distributed RTD 10 may be as small as approximately 0.003inch, and the wall thickness for the swaged and/or drawn outer sheath 12may be as little as approximately 0.002 inch.

The form of distributed RTD shown in FIGS. 1-3 which does not include aheater filament in association with RTD wire filament 20 is an accuratelinear thermometer particularly adapted to measure the averagetemperature of a nonhomogeneous or nonisothermal temperature field orzone. Examples of such fields or zones are a tank of liquid havingstratified temperatures, and a flow pipe or duct with nonuniformtemperature media flowing therethrough. One or more of the distributedRTDs 10 may be deployed across such a nonisothermal temperature field tosense the average temperature of the field. In a situation such as astratified tank of liquid where the stratification is normally in thehorizontal direction only, a single one of the distributed RTDs 10 maybe utilized in a vertically oriented deployment, or a plurality ofparallel vertically deployed distributed RTDs 10 may be employed. In asituation where the isothermal temperature field is more complex incross-section, not being regularly or predictably stratified, such asnonuniform temperature media flowing through a duct, a crossed matrix oftwo or more of the distributed RTDs 10 may be employed.

Although the nonheater-type distributed RTD of the invention has beenshown in FIGS. 1-3 and described with a single RTD wire filament 20having its lead wire terminations 22 and 24 at opposite ends of itslength, it is to be understood that the nonheater form of the inventionmay have a plurality of parallel RTD wire filaments which areelectrically connected in series or parallel, and which may terminate atthe same end of the distributed RTD when connected in series. Suchmultiple RTD filament arrangements are illustrated in FIGS. 8-11 of thedrawings, which show forms of the invention that also include heaterfilaments.

Although the form of distributed RTD shown in FIGS. 1-3 does not includea heater filament, the outer sheath when made of metal may be utilizedto provide the heating function, in which case the distributed RTD 10 ofFIGS. 1-3 may be employed in the same manner and for the same purposesas the heater-type forms of the invention shown in FIGS. 5-15 asdescribed hereinafter. In this case, the outer sheath will be made of aductile metal having a high electrical resistance, and preferably has aresistance which does not change as a function of temperature over theoperating temperature range of the distributed RTD, such as "Even-Ohm"material. Nichrome is also generally satisfactory for the purpose, andis particularly good for high temperature uses. In this case, theelectrical connections for the heating current made at ends 14 and 16 ofsheath 12. If this heatable distributed RTD is to be utilized for liquidlevel sensing in an electrically conductive liquid, the outer sheath ispreferably coated with a layer of insulating material to avoid shorting.

Referring now to FIGS. 5-7, these figures illustrate a heater-typedistributed RTD according to the invention which is generally designated28, and includes a pair of parallel filaments, one of which is an RTDwire filament and the other of which is a heater wire filament.Distributed RTDs of the invention which include a heater wire filamentare adapted for making several different types of measurements. Perhapsthe most widely useful deployment of heater-type distributed RTDs of theinvention is to provide continuous gauging of the level or location of aphase change interface; this usually being the liquid level or interfacebetween liquid and gas, but may also be a liquid-to-liquid interfacebetween nonmiscible liquids, or level of particulate matter in eitherliquid or gas. Another use of heater-type distributed RTDs of theinvention is to measure the average mass flow velocity of gas flow inlarge ducts where a nonuniform flow velocity distribution is caused byflow disturbances from the presence of such fittings as tees, elbows,transitions, bends, section changes, louvers, dampers, and the like.Average mass flow velocity of any fluid, whether gas or liquid, in anyconduit can be measured by heater-type distributed RTDs of theinvention. The manner in which heater-type distributed RTDs of theinvention are utilized to gauge liquid level and measure average flowvelocity in a duct will be described in detail hereinafter.

Heater-type distributed RTDs of the invention may also be used forsensing the average temperature of a field or zone in the same manner asdescribed above for the nonheater form of the invention shown in FIGS.1-3, by simply not electrically connecting the heater element orelements to a source of current or voltage.

The heater-type distributed RTD 28 of FIGS. 5-7 has outer sheath 30,preferably of metal, as its primary structural basis, sheath 30 havingends 32 and 34. Sheath 30 is filled, except for the presence of both anRTD wire filament and a heater wire filament, along its entire lengthbetween its ends 32 and 34 with uninterrupted body 36 of electricalinsulation material, which is preferably mineral. An RTD wire filament38 and a heater wire filament 40 extend through the entire length ofsheath 30 and insulation body 36, filaments 38 and 40 being in spaced,parallel relationship relative to each other, and both being spacedradially inwardly from the inner surface of sheath 30. Filaments 38 and40 are preferably equally radially outwardly spaced from the axialcenter of insulation body 36, and are electrically insulated from eachother and from the sheath 30 by the material of insulation body 36. Theends of distributed RTD 28 are preferably dressed to a squared-offshape, and RTD lead wires 42 and 44 are connected to the respective endsof RTD filament 38, lead wires 46 and 48 being similarly connected tothe respective ends of heater filament 40. The ends of distributed RTD28 of FIGS. 5-7 are preferably provided with moisture barriers such asseals 25 on the form of the invention shown in FIGS. 1-3.

The heater-type distributed RTD 28 of FIGS. 5-7 is manufactured in thesame manner as the distributed RTD 10 of FIGS. 1-3, starting with arelatively short and thick billet-like assembly similar to the assembly26, with the sheath, insulation body, RTD rod and heater rod in the samerelative diametral proportions as the final very long, very thin sheath30, insulation body 36, RTD wire filament 38, and heater wire filament40. RTD filament 38 and heater filament 40 may have diameters as smallas approximately 0.003 inch each, and the sheath 30 may have a wallthickness as small as approximately 0.002 inch. With the two filaments38 and 40, the OD of distributed RTD 28 will be somewhat larger than0.010 inch, but may still be as small as approximately 0.020 to 0.030inch or less.

The materials of which sheath 30, insulation body 36, and RTD wirefilament 38 are made are preferably as described hereinabove for thecorresponding components of distributed RTD 10 of FIGS. 1-3. Heater wirefilament 40 is made of nichrome or other suitable conventional metalheater material having relatively high electrical resistance and havingthe required ductility to be swaged and/or drawn to the very long, verythin configuration of distributed RTD 28. Distributed RTD 28 may be aslong as desired for a particular use, as for example 60-100 feet, oreven several hundred feet if desired.

FIGS. 8 and 9 illustrate a further heater form of the present inventionin which the distributed RTD, generally designated 50, has a pair of RTDwire filaments extending substantially the length of distributed RTD 50,and a single heater wire filament thermally associated with both ofthese RTD wire filaments. In the embodiment shown in FIGS. 8 and 9, theRTD lead wire terminals come out of one end of distributed RTD 50, theother ends of the RTD wire filaments being interconnected so that thefilaments are arrayed in series. This arrangement has severaladvantages. First, the double RTD material length afforded by the twoRTD lead wires connected in series provides additive response with twicethe sensitivity of a single RTD wire filament, and hence twice theresolution and accuracy capability for the information being sensed.Another important advantage of having the two RTD wire filamentsarranged in series with their lead wires at the same end of distributedRTD 50 is that electrical connection to detection circuitry and currentor voltage sources need only be made at one end of distributed RTD 50,which is much more convenient and involves much less conductor lengththan if such connections are required at both ends of the distributedRTD. For example, if distributed RTD 50 is vertically oriented throughthe height of a tank for continuous gauging of liquid level, then theelectrical connections to detection circuitry and current sources can bemade entirely from the top of distributed RTD 50, and need not come outthrough a wall of the tank or all of way up from the bottom to the topof the tank, as would be the case where connections were required atboth ends of the distributed RTD. In the form 50 of the invention shownin FIGS. 8 and 9, the heater wire filament is shown with lead wireconnections at both ends of distributed RTD 50. However, the heaterfilament need only receive current or voltage at one end, which will bethe same end as has the two RTD filament lead wire terminals, and theother end of the heater wire filament may be electrically grounded tothe sheath or elsewhere if desired.

Outer sheath 52, preferably of metal, forms the primary structural basisfor distributed RTD 50, and has ends 54 and 56. Sheath 52 is filledbetween its ends 54 and 56 with insulation body 58, preferably ofmineral insulation material, which is uninterrupted except for thepresence of the pair 60 and 62 of spaced, parallel RTD filaments, andheater filament 64. Heater filament 64 is coaxially centrally locatedwithin the cylindrical insulation body 58 and sheath 52, and RTDfilaments 60 and 62 are arranged parallel to heater filament 64 andequally radially outwardly spaced from heater filament 64. Thus, each ofthe RTD filaments 60 and 62 is electrically insulated from heaterfilament 64 and from each other, while at the same time, each of the RTDfilaments 60 and 62 is thermally coupled to heater filament 64 throughthe material of insulation body 58.

The adjacent ends of the two RTD wire filaments 60 and 62 areelectrically connected to each other proximate one end 56 of sheath 52,by means of a connection 65 which is preferably a welded connection.This connection 65 is preferably disposed within the sheath 52 adjacentits end 56 so that it can be covered with insulation material. This isaccomplished by scooping out some of the material of insulation body 58from within sheath end 56, effecting the connection 65 and arranging theconnected end portions of RTD filaments 60 and 62 as best seen in FIG. 9so that they are spaced from heater filament 64, and then providing abackfill 66 of insulation material out flush to sheath end 56 as shownin FIG. 8.

At the opposite end of distributed RTD 50 proximate sheath end 54, RTDfilaments 60 and 62 are provided with respective lead wires 68 and 70.The opposite ends of heater filament 64 are provided with respectivelead wires 72 and 74. The ends of distributed RTD 50 of FIGS. 8 and 9are preferably provided with moisture barriers such as seals 25 on theform of the invention shown in FIGS. 1-3.

Distributed RTD 50 is manufactured in the same manner and with the samematerials as described in connection with distributed RTDs 10 and 28 ofFIGS. 1-7, and has the same very long, very thin overall configuration,being manufacturable in lengths up to several hundred feet as requiredfor the particular region being sensed. The OD of distributed RTD 50 maybe as small as approximately 0.020 to 0.030 inch.

FIG. 10 shows a distributed RTD, generally designated 76, which hasgenerally the same construction as distributed RTD 50 shown in FIGS. 8and 9, except that it has four RTD wire filaments instead of two, thefour being connected together in series to present only two RTD filamentends at one end of distributed RTD 76. The four-RTD element distributedRTD 76 of FIG. 10 has the same advantage as the two-RTD elementdistributed RTD of FIGS. 8 and 9 in having its two RTD lead wireterminals located at the same end of distributed RTD 76, but has thefurther advantage of twice the sensitivity, and hence twice the read-outresolution and accuracy, as the two-RTD wire distributed RTD 50 of FIGS.8 and 9, and four times the sensitivity and read-out resolution andaccuracy as the single RTD filament forms 10 of FIGS. 1-3 and 28 of FIG.5-7.

Distributed RTD 76 illustrated in FIG. 10 has outer sheath 78,preferably of metal, with ends 80 and 82, and continuous insulation body83, preferably of mineral insulation material, extending substantiallythe entire length of sheath 78. The four RTD wires are designated 84,86, 88 and 90, and are all parallel and uniformly spaced aroundinsulation body 83, being equally radially spaced outwardly from thecentral longitudinal axis of insulation body 83. A single heater wire 92is coaxially centered in insulation body 83, extending the length ofdistributed RTD 76. With this arrangement, the four RTD wires 84, 86, 88and 90 are radially equidistant from heater wire 92 so as to have thesame thermal coupling with heater wire 92 through the insulationmaterial of body 83.

The ends of RTD wires 84 and 86 adjacent end 82 of sheath 78 areconnected at 94, preferably by welding. The ends of RTD wires 86 and 88are connected, preferably by weld 96, adjacent end 80 of sheath 78. Theends of RTD wires 88 and 90 are connected, preferably by weld 98,adjacent the end 82 of sheath 78. In this manner, the four RTD wirefilaments 84, 86, 88 and 90 are arranged as a series circuit.Considering sheath end 80 to be at the front end of distributed RTD 76and sheath end 82 to be at the rear end of distributed RTD 76, thisseries circuit starts at an exposed front end of RTD filament 84extending to proximate the rear end through filament 84, then extendingthrough RTD filament 86 back forwardly to proximate the front end, thenextending back rearwardly through RTD filament 88 to proximate the rearend, and then extending back forwardly through RTD filament 90 which isexposed at the front end.

The rear ends of filaments 84 and 86 and their connection 94, and therear ends of filaments 88 and 90 and their connection 98, are recessedwithin the rear end 82 of sheath 78, and covered with insulationbackfill 99. Similarly, the front ends of filaments 86 and 88 and theirconnection 96 are recessed within the front end 80 of sheath 78 andcovered with insulation backfill 100. The exposed front ends of RTDfilaments 84 and 90 are provided with respective RTD lead wires 102 and104, and the exposed front and rear ends of heater filament 92 areprovided with respective lead wires 106 and 108. The ends of distributedRTD 76 of FIG. 10 are preferably provided with moisture barriers such asseals 25 on the form of the invention shown in FIGS. 1-3.

Distributed RTD 76 of FIG. 10 is made with the same materials and by thesame procedures as described hereinabove in connection with other formsof the invention, and may be made as long as the other forms, and asthin as the distributed RTD form 50 illustrated in FIGS. 8 and 9.

FIG. 11 illustrates still another form of the invention wherein both RTDand heater filaments are provided in pairs so as to enable allelectrical connections to be made at one end of the distributed RTD.Distributed RTD 110 shown in FIG. 11 has the usual sheath of theinvention, designated 112, preferably of metal, with front and rear ends114 and 116, respectively. Insulation body 118, preferably of mineralinsulation material, extends through the length of sheath 112. A pair ofspaced, parallel RTD wire filaments 120 and 122 extend longitudinallythrough insulation body 118, and their rear ends are connected at 124,which is preferably a welded connection. RTD lead wires 126 and 128extend from the front ends of respective RTD filaments 120 and 122.Similarly, a pair of spaced, parallel heater wire filaments 130 and 132extend longitudinally through insulation body 118 and are connected attheir rear ends at connection 134, which is preferably a weldedconnection. Heater lead wires 136 and 138 extend from the front ends ofrespective heater wires 130 and 132. RTD filaments 120 and 122 andheater filaments 130 and 132 are all radially equidistant from thelongitudinal axial center of insulation body 118 and sheath 112, and RTDfilaments 120 and 122 and heater filaments 130 and 132 are alternatelyuniformly spaced about insulation body 118 so that each of the twoheater filaments 130 and 132 has the same thermal coupling and effect oneach of the RTD filaments 120 and 122. The rear ends of RTD filaments120 and 122 and their connection 124, and the rear ends of heaterfilaments 130 and 132 and their connection 134 are recessed inside therear end of sheath 112, and are covered with insulation backfill 140 andalso preferably an end plug 142 which is preferably welded in the rearend 116 of sheath 112 to provide a moisture barrier. The front end ofdistributed RTD 110 of FIG. 11 is preferably also provided with amoisture barrier, such as one of the seals 25 on the form of theinvention shown in FIGS. 1-3.

Distributed RTD 110 of FIG. 11 is made with the same materials and bythe same procedures as described hereinabove in connection with otherforms of the invention, and may be made as long as the other forms, andas thin as the distributed RTD forms of FIGS. 8-10.

The forms of the invention shown in FIGS. 1-11 may all be considered tobe linear forms of distributed RTDs.

Although it is preferred to employ internal heater filaments in theforms of the invention shown in FIGS. 5-11, the heater for any of theseembodiments may alternatively be the outer sheath where it isconductive, as of metal, in the manner described above for theembodiment of FIGS. 1-3.

Although the forms of the invention shown in FIGS. 8-11 which have aplurality of RTD filaments show these electrically connected in series,it is to be understood that a plurality of RTD filaments mayalternatively be electrically connected in a parallel array, or ifdesired, they may each be independently electrically connected todetection circuitry, or may serve as redundant sensors.

FIGS. 12 and 13 illustrate another form of the invention in which theeffective length of RTD filament is greatly increased for increase ofsensitivity and resolution by arranging RTD filaments in a long, thin,helical array. The distributed RTD of FIGS. 12 and 13 is generallydesignated 144, and consists of an elongated axial inner heater coreportion generally designated 146 and an elongated helical distributedRTD peripheral portion generally designated 148.

Referring at first to heater core portion 146, it is defined within acylindrical sheath 150, preferably of metal, extending substantially theentire length of distributed RTD 144. Heater sheath 150, in addition toserving as a strengthener and support for the contained insulation bodyand heater filaments, also serves as a mandrel upon which a very long,thin, linear sheathed distributed RTD of the invention is helicallywound. Helical distributed RTD 144 is illustrated in FIG. 12 with itslongitudinal axis vertically arranged, which is the arrangement in whichit is deployed when utilized to gauge liquid level in a tank. Since thelead wires are connected at the upper end, this end may be considered asthe front end, while the lower end may be considered as the rear end.Heater core sheath 150 has an upper end 152 which registers with theupper end of distributed RTD 144, and a lower end 154 which is justinside the lower end of distributed RTD 144. Heater sheath 150 is filledalong its entire length with insulation body 156, which is preferably ofmineral insulation material, and a pair of spaced, parallel heaterfilaments 158 and 160 is supported in insulation body 156 alongsubstantially its entire length. Heater filaments 158 and 160 areconnected at their lower ends by connection 162, preferably a weldedconnection, which is inside the lower end portion of sheath 150.Insulation backfill 164 covers the connected lower ends of heaterfilaments 158 and 160, and a plug 166 at the bottom of heater sheath 150covers the backfill 164. The upper ends of heater filaments 158 and 160are exposed at the upper end of insulation body 156, and have respectivelead wires 168 and 170 connected thereto.

Heater core portion 146 is made according to the procedure heretoforedescribed in detail with respect to nonhelical or linear forms of theinvention, commencing with a relatively short, thick starting assemblyor billet, and then swaging and/or drawing it, with annealing steps asrequired, to its long, thin final configuration. The materials of heatersheath 150, insulation body 156, and heater filaments 158 and 160 arethe same as those described hereinabove in connection with linear formsof the invention.

Helical peripheral portion 148 of distributed RTD 144 is a linear formof the invention similar to those previously described which isextremely long and very thin so as to be adaptable for helical windingon the mandrel furnished by heater sheath 150. Helical RTD peripheralportion 148 shown in FIGS. 12 and 13 is a sheathed distributed RTDgenerally designated 172 which has a pair of RTD wire filamentsextending its length which are exposed at one end and connected andsealed in insulation material at the other end. Thus, distributed RTD172 of FIGS. 12 and 13 may be the same as distributed RTD 50 illustratedin FIGS. 8 and 9 and described in detail in connection therewith, exceptfor the absence of heater filament 64 of FIGS. 8 and 9, such heaterfilament not being necessary in distributed RTD 172 because of heatercore 146 in helical distributed RTD 144. The already great length of RTDfilament enabled by the helically wound configuration in distributed RTD144 is thus doubled by the use of a pair of RTD wire filaments indistributed RTD 172, and having the pair of RTD filaments enables allelectrical connections to be made at the upper end of helicaldistributed RTD 144. Alternatively, RTD filament length may bequadrupled by utilizing the RTD filament arrangement of FIG. 10.

The very long, linear distributed RTD 172 wound in the helix has outersheath 174, preferably of metal, as its primary structural basis, thesheath having front, upper end 176 and rear, lower end 178. Sheath 174is filled along its entire length with insulation body 180, preferablymineral, within which spaced, parallel RTD wire filaments 182 and 184are supported. The lower, rear ends of RTD filaments 182 and 184 areconnected at 186, which is preferably a welded connection, and arecovered with insulation backfill 188 and end plug 190. The upper, frontends of RTD filaments 182 and 184 are exposed at the corresponding endof insulation body 180, and are provided with respective lead wires 192and 194.

Linear distributed RTD 172 is made in the same way and with the samematerials as the other linear RTDs of the invention.

Linear distributed RTD 172 is preferably closely helically wound aboutheater sheath 150 with adjacent coils touching as illustrated in FIG. 12to obtain a maximum amount of RTD filament length. However, if desired,overall RTD filament length may be varied by spacing the adjacent coilsapart any selected amount. Normally, the spacing between adjacent coilswill be the same along the entire length of helical distributed RTD 144to provide a linear gauging or sensing function. However, the gauging orsensing function may be made variable along the length of helicaldistributed RTD 144 by varying the spacing between adjacent coils alongthe length of helical distributed RTD 144. Such a variable functionhelical distributed RTD will be described hereinafter in connection withthe diagramatic illustration of FIG. 19.

A cylindrical outer metal protective jacket 196 is coaxially disposedover coiled distributed RTD 172 along the entire length of helicaldistributed RTD 144, and the lower end of jacket 196 is covered with abottom plug 198 to protect the lower end of the coil. Jacket 196 is madeout of the same type of ductile metal as the outer metal sheathes of thelinear forms of the invention, and is swaged and/or drawn into a tightfit over the coiled distributed RTD 172.

Helical distributed RTDs 144 will typically be made in lengths up toapproximately 60 feet or longer for gauging liquid levels in tanks, andgenerally will be much shorter than that for measuring average flowvelocity in a duct. The coiled linear distributed RTD 172 may have an ODas small as approximately 0.015 inch, which enables helical distributedRTD 144 to have an overall OD as small as approximately 1/8th inch.Helical distributed RTD 144 is thus still thin enough to be rolled upfor convenience of storage, shipping, and handling in close quarters.The length of helical distributed RTD 144 will usually be severalhundred times the OD. Several hundred feet of linear distributed RTD 172which is helically wound is preferred for extremely high sensitivity andresolution of helical distributed RTD 144.

Although the helical form of distributed RTD has been shown in FIGS. 12and 13 and described in detail hereinabove utilizing a heater core 146and no heater filaments in the coiled linear distributed RTD 172, it isto be understood that the heater core may be omitted and alternativelyone of the heater filament types of linear distributed RTDs of theinvention employed as the coiled element. In such a helical distributedRTD, a cylindrical core like cylindrical core member 150 will serve as amandrel for the coil, and internal insulation body 156 and heaterfilaments 158 and 160 omitted. The preferred coiled linear RTD elementwould then be the form illustrated in FIG. 11, which has self-containedheater filaments 130 and 132 along with the two RTD filaments 120 and122, all four filaments having their lead wires coming out the same endof this form 110 of linear distributed RTD.

FIG. 14 illustrates another variation 144a of helical distributed RTDaccording to the invention in which the helically coiled linear RTDelement is encased in a suitable potting or filler material such asepoxy resin as a protection against physical and chemical damage. Thisfiller material may, if desired, be a suitable metal. Such potting orfiller material will serve the same protective function as outer metaljacket 196 in the form shown in FIGS. 12 and 13, although it, too, mayalternatively have the additional protection of an outer metalprotective jacket of the type shown in FIGS. 12 and 13 for even furtherprotection.

The other components of helical distributed RTD 144a of FIG. 14 are thesame as the corresponding components of helical distributed RTD 144 ofFIGS. 12 and 13. Thus, helical distributed RTD 144a has a heater coreportion 146a and a helical distributed RTD peripheral portion 148a, coresheath 150a extending from an upper end down to a lower end 154a, withlinear distributed RTD 172a coiled about the length of cylinder 150a.The generally rigid potting or filler material 200 fills the spacesbetween all of the adjacent coils of linear distributed RTD 172a alongthe entire length of helical distributed RTD 144a to provide a generallysolid protective body. Despite the general rigidity of the potting orfiller material 200, the very thin OD of helical distributed RTD 144a,as small as approximately 1/8th inch, nevertheless gives helicaldistributed RTD 144a the same spring-like flexibility as helicaldistributed RTD 144, enabling it to be rolled up for convenience.

FIG. 15 shows a further variation of helical distributed RTD accordingto the invention, wherein two layers of helically wound lineardistributed RTDs of the invention are provided to double overall RTDfilament length, and thereby double the sensitivity and resolutionrelative to the helical distributed RTD of FIGS. 12 and 13. The doublehelix form of distributed RTD of FIG. 15 is generally designated 144b,and is the same as helical distributed RTD 144 in all respects exceptthe additional helically wound linear distributed RTD. Thus, doublehelix distributed RTD 144b has heater core portion 146b and doublehelical distributed RTD portion 148b. Core portion 146b has outercylindrical metal sheath 150b, with an inner linear distributed RTD 172bof the invention coiled directly around sheath 150b, and an outer lineardistributed RTD 202 helically wound around the outside of inner linearRTD 172b. By having two separate linear distributed RTDs 172b and 202,they are enabled to be coiled in the same direction so that the coils ofthe outer helix nest in the grooves between the coils of the inner helixfor diametral compactness.

An outer metal protective jacket 196b is swaged and/or drawn over theoutside of outer linear distributed RTD 202 along the entire length ofthe double helix distributed RTD 144b; and if desired, as an alternativeor supplemental protective measure, both of the helical coils may beencased in the rigid potting or filler material of FIG. 14. Lead wires192b and 194b connect to the two RTD filaments of inner lineardistributed RTD 172b, and lead wires 204 and 206 connect to the RTDfilaments of outer linear distributed RTD 202.

Despite the additional helical layer, the double helix distributed RTD144b still can be as small as approximately 0.150 inch in OD, and has aspring-like flexibility enabling it to be rolled up.

Another way of increasing the effective length of RTD filament forincreasing sensitivity and read-out resolution is to bundle a pluralityof any of the long, thin distributed RTDs of the invention together in aparallel array within an outer jacket like jacket 196 of FIG. 12 or 196bof FIG. 15, and swage or draw the jacket down on the bundle to hold itfirmly together. Corresponding lead wires of distributed RTDs may beconnected together in series proximate the ends of the distributed RTDsso as to multiply the overall sensitivity of the bundle and alsominimize the number of wires leading to detection circuitry. They mayalternatively be connected in parallel, or may be individually connectedto detection circuitry. They may also serve as redundant sensors.

Referring to FIG. 16, distributed RTDs of the heater types according tothe invention are deployed in matched parallel, vertical pairs forliquid level sensing. These are preferably of the type wherein bothheater filaments and RTD filaments have all of their electricalterminations at one end. Thus, the forms of the invention shown in FIGS.11-15 are preferred for this purpose.

Such a matched pair of distributed RTDs is designated 208 and 210, andfor maximum sensitivity and resolution, will be assumed to be either apair of distributed RTDs 144 of FIGS. 12 and 13, 144a of FIG. 14, or144b of FIG. 15. The pair of distributed RTDs 208 and 210 is deployedvertically in closely spaced, parallel relationship inside a tankgenerally designated 212 having side walls 214, a bottom wall 216, and atop wall 218. A series of mounting brackets 220 supports the twodistributed RTDs 208 and 210 within tank 212, spaced inwardly from themounting wall 214. The upper end portions 222 of distributed RTDs 208and 210 extend upwardly beyond top wall 218 through a panel or plugmember 224 set in top wall 218. This makes the lead wire bundles 226 ofthe two distributed RTDs 208 and 210 accessible above top wall 218. Thelower ends 228 of distributed RTDs 208 and 210 may be disposed asclosely as desired to the inside of bottom tank wall 216.

To operate the pair 208 and 210 of distributed RTDs for gauging liquidlevel in tank 212, the heater filament or filaments of one of thedistributed RTDs 208 and 210 are energized, while the heater filament orfilaments of the other distributed RTD are left unenergized. Assumingthere is quiescent liquid in the tank up to a level such as thatdesignated by phantom horizontal line 229, and still air in the rest ofthe tank above line 229, the relatively high density liquid willdisperse heat away from the outer sheath of the heated distributed RTDmuch more efficiently than the relatively poor dispersion rate of air.Preferably, sufficient current or voltage is supplied to the heaterfilaments of the heated distributed RTD to heat the portion of it thatis in air to a differential temperature of approximately 100° F. abovethe temperature of the unheated distributed RTD, which serves as areference. The much more efficient thermal conductivity of the quiescentliquid will cool off the immersed portion of the heated distributed RTDto a differential temperature on the order of only about 10° F. abovethe unheated reference distributed RTD. As the liquid level 229progressively rises in tank 212, a progressively greater length of theheated RTD is cooled off to the approximately 10° F. differential and aprogressively shorter length of the heated RTD will still have the high,approximately 100° F. differential. Conversely, as liquid level 229lowers in tank 212, a progressively greater length of the heated RTDwill be uncovered above liquid level 229 and therefore subject to thehigh temperature differential of approximately 100° F., and aprogressively shorter length of the heated RTD below liquid level 229will be cooled to the low, approximately 10° F. temperaturedifferential. The resulting continuous variation of electricalresistance in the heated RTD relative to the electrical resistance inthe unheated RTD according to variations in liquid level in tank 212will permit good accuracy, high resolution, smooth, continuous, linear,nonstepped gauging and analog reading of liquid level in the detectioncircuitry and instrumentation electrically connected to lead wirebundles 226 of the two continuous RTDs 208 and 210. Such high accuracyand resolution, continuous gauging not only enables the exact liquidlevel to be determined at any time, but also enables liquid levelchanges to be immediately sensed, as compared with prior art pointsensing systems which could allow a considerable liquid level change tooccur prior to detection. This also avoids the large expense associatedwith the many connections required to be made when point sensors areemployed.

In the event the liquid and/or air in the gauged vessel may be turbulentrather than quiescent, then greater cooling of heated distributed RTD208 or 210 will be caused by the turbulence, and the aforementioned 10°F. and/or 100° F. temperature differential correspondingly decreased. Ifthe turbulence is constant, the system can be calibrated in an equallyturbulent environment, and provide readouts of satisfactory accuracy. Onthe other hand, if the turbulence is randomly varying such that variablevalues of cooling occur, the system can produce errors in the readoutcaused by the random turbulence. If such is the case, then a still wellor stilling well can be provided which surrounds at least the heateddistributed RTD, and preferably both the heated and unheated distributedRTDs so that it can also serve as a support for both, to mitigate theeffects of the varying turbulence. Such a still well is illustrated inFIGS. 24 and 25 and described hereinafter in connection therewith.

FIGS. 17 and 18 illustrate use of a matched pair of distributed RTDsaccording to the invention to measure the average mass flow in a ductwhere a flow of nonuniform distribution exists. Such nonuniform flowdistributions exist, for example, in air ducts where diameters are largeand fittings such as tees, elbows, transitions, bends, section changes,louvers, dampers, and the like cause flow disturbances. Another exampleof a large duct where it would be desirable to measure the average massflow velocity or rate is the input air duct to and/or the combustionproducts output duct from a fossil fuel power plant. The matched pair ofdistributed RTDs in FIGS. 17 and 18 is designated 230 and 232, and theseare heater type RTDs according to the invention, preferably one of thetypes of FIGS. 11-15 which have electrical connections at one end. Thepair of distributed RTDs 230 and 232 is shown deployed in spaced,parallel relationship to each other diametrically across an air ductelbow 234 in the direction from the smallest radius of curvature at theinside 236 of the duct bend to the largest radius of curvature at theoutside 238 of the duct bend, this being the direction of greatestnonuniformity of flow distribution but symmetrical in the normal axis.

The front end portions 240 of distributed RTDs 230 and 232 which carrythe lead wires extend externally of the wall of the duct through asuitable support body 242 in the region of the outer curvature 238 ofthe elbow, with lead wire bundles 244 accessible for connection to thedetection circuitry and current or voltage source. The rear end portions246 of distributed RTDs 230 and 232 are secured in a suitable internalsupport body 248 in the region 236 of the inner curvature of the elbow.

The heater filaments of one of distributed RTDs 230 or 232 areenergized, and the heater elements of the other are not, the unheateddistributed RTD serving as a reference. The flowing air cools the heateddistributed RTD as a function of the mass flow rate passing the heatedRTD. Relatively high flow rates near the longer outer curvature region238 of the elbow will cool that portion of the heated RTD to a greaterdegree than the cooling that will occur near the shorter inner curvatureregion 236 of the elbow, and the resulting sensed output of RTDfilaments in the heated distributed RTD as compared to the sensed outputof RTD filaments of the unheated distributed RTD will provide anexcellent summation and average of the mass flow rate passing thematched pair of distributed RTDs 230 and 232.

If additional flow rate information is desired for the summing andaveraging of the flow rate in duct elbow 234, one or more additionalmatched pairs of spaced, parallel heated distributed RTD units like 230and 232 may be deployed across elbow 234, in close to the sametransverse plane across elbow 234, but offset angularly as viewed inFIG. 18 from the matched pair 230 and 232 of distributed RTDs, or ifdesired, a gridwork of crossing matched pairs of distributed RTDs like230 and 232 might be deployed across duct elbow 234.

Should the mass flow rate be so high as to cool heated distributed RTD230 or 232 to such a degree as to saturate its signal output, i.e.,where increased flow rate causes little or no additional cooling, ashroud can be provided to increase the usable range. The shroud reducesthe rate at which heat is dispersed from the heated distributed RTD intothe flowing stream. Such a shroud is illustrated in FIGS. 26 and 27, anddescribed in connection therewith.

Distributed RTDs 230 and 232 illustrated in FIGS. 17 and 18 are shownlinearly arranged diametrically across duct elbow 234. However, thecross-sectional area of duct 234 varies nonlinearly in the direction ofthe axis of distributed RTDs 230 and 232 (the vertical axis in FIG. 18).In order to provide a signal output truly representative of mass flowrate where flow is symmetrical about a diametrical axis as in FIG. 18,the distributed RTDs should be configured such that each incrementallength thereof is proportional to the corresponding incremental area ofthe duct in which it lies; that is, the distributed RTDs should beconfigured so as to respond as a variable function that is proportionalto duct area along their lengths. This can be accomplished in the samemanner as shown and described with reference to FIG. 19 for liquidvolume gauging with reference to a right circular tank on its side. Thiscan alternatively be accomplished by a sinuous or serpentineconfiguration of linear forms of the invention in the mannerdiagrammatically illustrated in FIG. 28 and described in connectiontherewith.

Where duct flow is symmetrical about the center axis of the duct,distributed RTDs of the invention can be arrayed in a spiralconfiguration such that incremental lengths thereof are proportional tocorresponding incremental circular flow areas as diagramaticallyillustrated in FIG. 29 and described in connection therewith.

Utilization of a matched pair of heater-type distributed RTDs of theinvention in each of the liquid level gauging and mass flow rate sensingserves two purposes. First, the unheated distributed RTD, by havingidentical components as the heated distributed RTD, has an identicalthermal response to the environment other than that response which isthe result of energizing the heater, so that a true differentialtemperature response is provided by comparison of the output of the RTDfilaments of the heated distributed RTD relative to the output of theRTD filaments of the unheated distributed RTD. Second, with the twomatching distributed RTDs, if the heater should fail in one of them,then the heater of the other one may alternatively be energized. In thismanner, only the external electrical connections need be changed, whichcan be done at the location of the detection circuitry, and then theformerly heated distributed RTD will serve as the reference distributedRTD, and the difficulty and expense of replacing the damaged distributedRTD is avoided.

FIG. 19 diagramatically illustrates a variable function type ofdistributed RTD embodying the principles of the present invention. Inthe situation depicted in FIG. 19, a right circular cylindrical tank 250is laid on its side, and it is desired to obtain a linear signal outputindicating the volume of liquid contained in tank 250 from a matchedpair 252 and 254 of spaced, parallel, vertically deployed heater-typedistributed RTDs of the invention. The problem here is that with thecylindrical tank on its side, volume does not increase proportionallywith liquid level so that linear function distributed RTDs of theinvention, while indicating liquid level, would not indicate volume. Theother forms of the invention described hereinabove in connection withFIGS. 1-15 are continuous linear function distributed RTDs. When tank250 is being filled from its empty condition, as the liquid level firstrises from the arcuate bottom of the tank, there is only a relativelysmall volumetric increase corresponding to liquid level height increase.As the liquid level rises further and further toward the vertical centerof the tank, the amount of liquid volume increase per increment ofliquid level height increase will become greater and greater until it isat a maximum at the vertical center of the tank. Conversely, as theliquid level further increases from the vertical center of the tank, theincrements of volume increase per increment of height increase willgradually be reduced at first, and then will be reduced at a greater andgreater rate, until only very small volume increases will correspond toincrements of added liquid level height as the arcuate top of the tankis approached.

To automatically compensate for this variable function of volumerelative to liquid level height in cylindrical tank 250 and provide anelectrical output from the matched pair of distributed RTDs 252 and 254which will provide a linear reading of liquid volume to the detectioncircuitry in the same manner as the matched pair 208 and 210 ofdistributed RTDs will for tank 212 in FIG. 16 which has a uniformhorizontal cross-section for its entire vertical height, each of thematching distributed RTDs 252 and 254 in FIG. 19 is of the helicallywound type, but has the coils of the winding variably separated alongthe length of the distributed RTD, with the winding variationscharacterized so as to represent the curvature of tank 250. Thus, eachof the matched distributed RTDs 252 and 254 is of the type shown inFIGS. 12 and 13, or in FIG. 14, or in FIG. 15, but the helically woundlinear heater-type distributed RTD 256 of each has its adjacent coilsclosely spaced proximate the vertical center of tank 250, and then thespacing gradually increases both upwardly and downwardly from thevertical center, and the increased spacing between adjacent coilsbecomes greater and greater as the top and bottom of the tank areapproached, being greatest adjacent both the top and bottom of the tank.The coil spacing variation from the bottom to the top of tank 250 isarranged to register with the liquid volumetric variations relative toliquid level height, to provide the linear output of distributed RTDsensors 230 and 232 which indicates liquid level volume in tank 250.

It will be apparent from the diagramatic illustration of FIG. 19 and theforegoing description relating to FIG. 19 that helical-type distributedRTDs of the invention may have coil spacings varied in any desiredmanner along their lengths to accomodate or compensate for any desiredfunction, or to produce any desired output function. Alternatively, thenonlinearity or discontinuity of any tank shape or other fieldirregularity can be accommodated with a linear distributed RTD of theinvention and electronic means such as microprocessor programing whichconverts the linear signal from the sensor into an actual volumetric orother desired field function that takes into account any nonlinearity ordiscontinuity in tank shape or other field irregularity.

FIG. 20 diagrammatically illustrates a helical winding arrangement forhelical distributed RTDs of the invention which will produce a stepfunction output. In FIG. 20, the liquid level in a tank 258 is beingsensed by a pair of matching distributed RTDs 260 and 262 arrangedvertically in spaced, parallel relationship. Each of the distributedRTDs 260 and 262 is of the heater type of the invention, and employs avery long, thin linear-type RTD of the invention which has alternatinghelically wound sections 264 and straight, axial sections 266 along itslength. Helically wound sections 264 and straight sections 266 of thetwo distributed RTDs 260 and 262 are respectively in vertical registry.Other than the alternating helical and straight sections 264 and 266,respectively, the matching distributed RTDs 260 and 262 may be made inaccordance with the structures of FIGS. 12 and 13, or of FIG. 14, orwith the use of two overlapping coiled linear RTDs of the invention,according to the structure of FIG. 15. The liquid level sensing outputof the pair of distributed RTDs 260 and 262 will be a stepped signalbecause of the very high response sensitivity of helical sections 264relative to the response sensitivity of straight, axial sections 266.

Although step function-type distributed RTDs 260 and 262 are preferablyemployed as a matched pair with one serving as a reference, a single oneof the distributed RTDs 260 or 262 may be employed for liquid levelsensing by identifying and counting the discrete steps as liquid levelrises from the bottom of the tank, and identifying and counting thesteps as liquid level drops back down. By thus being able to senseliquid level with only a single long, thin distributed RTD probeaccording to the invention, access can be gained into regions which maybe too constricted for a matched pair of distributed RTDs. An example ofsuch a region would be certain points in some nuclear reactor guidetubes.

The helical sections or clusters 264 are shown regularly spaced and ofuniform lengths along the lengths of distributed RTDs 260 and 262 inFIG. 20, which will produce a linear step function output signal. If anonlinear step function output signal is desired, then a nonlinear orirregular spacing may be provided between helical sections 264, orhelical sections 264 may be provided with irregular lengths.

As an alternative to the alternating helical and straight sections ofdistributed RTDs 260 and 262 shown in FIG. 20, linear-type distributedRTDs of the invention may be configured to have alternating curvedsections such as transversely directed hairpin-shaped sections andstraight sections to produce the step function response.

Another way of providing a step function output for distributed RTDsaccording to the invention is illustrated in FIG. 21 which, forsimplicity of disclosure, illustrates the concept in connection with atwo-filament distributed RTD embodiment similar to that shown in FIGS.5-7. The distributed RTD of FIG. 21 is generally designated 268, and hasouter sheath 270, preferably of metal, inner insulation body 272,preferably of mineral insulation material, and RTD and heater filaments274 and 276, respectively, extending in spaced, parallel relationshipthrough the length of insulation body 272. RTD filament 274 iscontinuous as in the form of the invention shown in FIGS. 5-7. However,heater filament 276 is segmented into alternate high resistance sections278 and low resistance sections 280 so as to provide the step function.The high resistance sections 278 may be of nichrome or other highresistance metal, while the low resistance sections 280 may be of copperor other low resistance metal. The alternating portions of RTD filament274 adjacent high resistance heater filament sections 278 will be heatedto a much greater temperature than the alternating portions of RTDfilament 274 adjacent low resistance heater filament sections 280. Theheated portions of RTD filament 274 will have a high degree ofsensitivity to immersion in liquid, being cooled from perhaps 100° F.above ambient temperature down to perhaps 10° F. above ambienttemperature, while the unheated portions of RTD filament 274 will haveonly minimal response to liquid immersion. Thus, the output ofdistributed RTD 268 will be in the form of a step function. As liquidlevel rises, each heated section of RTD filament 274 will beincrementally cooled and thus have a lowered electrical resistance todefine the upward steps; while as liquid level drops, the heatedsections of RTD filament 274 which are uncovered will become hot andtheir elevated electrical resistance will define the successive downwardsteps.

As with the form of the invention shown in FIG. 20, distributed RTDs 268are preferably deployed in matched pairs for liquid level sensing, withonly one of them having its heater filament 276 energized so that theother serves as a reference, but alternatively a single one ofdistributed RTDs 268 may be employed for liquid level sensing byidentifying and counting the discrete steps as liquid level rises fromthe bottom of the tank and as liquid level drops back down.

As an alternative to the continuous RTD filament 274 and segmentedheater filament 276 described above for FIG. 21, the heater filament maybe provided in continuous form and the RTD filament may be segmentedhaving alternate segments that are of highly responsive RTD material,and segments that have low resistance or are unaffected (or lessaffected) by heat.

FIG. 22 shows a further form of distributed RTD according to theinvention which senses as a step function and correspondingly has a stepfunction output. Distributed RTD 282 of FIG. 22 is a two-filamentdistributed RTD which is like distributed RTD 28 of FIGS. 5-7 except forhaving a segmented insulation body which provides the step function.Thus, distributed RTD 282 has outer sheath 284, preferably of metal,insulation body 286, preferably of mineral insulation materials, and RTDand heater filaments 288 and 290, respectively. Insulation body 286 isprovided with alternate sections 292 having high thermal conductivityand sections 294 having low thermal conductivity, the difference inthermal conductivity being provided by the use of different types ofinsulation material for the sections 292 and 294. In this embodiment,heater filament 290 and RTD filament 288 are continuous, but thesections of RTD filament 288 within high thermal conductivity insulationsections 292 will be heated to a higher temperature than the sections ofRTD filament 288 in low thermal conductivity insulation sections 294.Thus, the sections of RTD filament 288 in high thermal conductivityinsulation sections 292 will have greater sensitivity than the sectionsof RTD filament 288 in low thermal conductivity insulation sections 292,providing the step function in the same manner as described above inconnection with distributed RTD 268 of FIG. 21. As with forms of theinvention shown in FIGS. 20 and 21, distributed RTD 282 of FIG. 22 ispreferably employed in a matched pair with heater filament 290 of onlyone of the pair energized so that the other will serve as a reference,but liquid level measuring can be accomplished with a single distributedRTD 282 by counting the incremental output steps.

RTD filaments of the invention may be energized by either a constantvoltage source or a constant current source, both of which are known tothose skilled in the art. With a constant voltage source applied acrossthe RTD filaments, the detection circuitry will be arranged to detectdecreases in current through the RTD filaments for increases intemperature sensed by distributed RTDs of the invention, and converselywill detect increases in current for decreases in temperature. With aconstant current source applied through the RTD filaments, the detectioncircuitry will be arranged to detect increases in voltage across the RTDfilaments for increases in temperature sensed by distributed RTDs of theinvention, and conversely will detect decreases in voltage for decreasesin temperature. For forms of the invention shown in FIGS. 1-15, suchcurrent and voltage responses for the respective constant voltage andconstant current source circuits will be smooth, continuous and linear.For the form of the invention diagramatically illustrated in FIG. 19,the response will have a variable function representative of thecurvature of the tank, but which will be a linear representation of theliquid level in the tank. For forms of the invention shown in FIGS.20-22, the response will be a step function which will be linear alongthe lengths of the distributed RTDs.

In forms of the invention shown in FIGS. 8-15 which have a plurality ofRTD filaments, these filaments have been shown and described as beingelectrically connected in series for additive response. This ispreferred for a constant current source system, as otherwise eachfilament would require separate electrical connection to the detectioncircuitry, where the voltage changes across the individual RTD filamentswould have to be added, instead of the voltage changes being added bythe series connections right in the distributed RTDs. The seriesconnection of the filaments will also work for a constant voltagesystem, provided the overall applied voltage is multiplied according tothe number of RTD filaments to keep the same sensitivity of eachfilament. However, with a constant voltage system, the RTD filaments mayalternatively be arranged in a parallel electrical array with minimalelectrical connections to the detection circuitry, and in such case, thecurrent change response to temperature change would still be additive,and the voltage would not have to be multiplied.

A simplified constant voltage-type detection circuit for use withmatched pairs of distributed RTDs of the invention, where one of thedistributed RTDs has its heater filament or filaments energized and theother distributed RTD has its heater filament or filaments unenergizedfor reference purposes, is shown in U.S. Pat. No. 3,366,942 to Deane andalso in U.S. Pat. No. 3,898,638 to Deane and McQueen.

A simplified constant current-type detection circuit for use inconnection with such matched pairs of distributed RTDs of the inventionis shown in block diagram form in FIG. 23. Although single RTD andheater filaments are shown in the simplified circuit of FIG. 23, it isto be understood that these diagramatically represent the RTD and heaterfilaments of any of the distributed RTDs shown in FIGS. 1-22 anddescribed in detail hereinabove.

The heated distributed RTD is generally designated 300, and includes RTDfilament 302 and heater filament 304 which is thermally coupled to RTDfilament 302. The unheated distributed RTD is generally designated 306,and includes RTD filament 308. The heater filament of unheateddistributed RTD 306 is not shown in the diagram since it is notelectrically connected to the heater power source, but it is to beunderstood that there is preferably a heater filament in unheateddistributed RTD 306 so that distributed RTD 306 is, for referencepurposes, an exact counterpart of heated distributed RTD 300. Heaterfilament 304 of heated distributed RTD 300 is electrically energizedthrough a conductor 309 by an electrical power source generallydesignated 310 which may be either a constant current source or aconstant voltage source.

Preferably a pair 312 and 314 of balanced precision constant currentsources, which have a power source 316, is electrically connectedthrough respective conductors 318 and 320 to one side of respective RTDfilaments 302 and 308, the other side being connected to ground. Theoutputs of respective RTD filaments 302 and 308 are electricallyconnected to the respective inputs of instrumentation amplifier 322,which is a differential operational amplifier, such connections beingmade through low resistance conductors 324 and 326 which lead fromrespective conductors 318 and 320 to amplifier 322. The output ofinstrumentation amplifier 322 is connected to the input of a signalprocessor 328, which may be any conventional microprocessor withadequate capacity, and the output of signal processor 328 is in turnconnected to output circuits 330 and 332 which drive suitableinstrumentation.

FIGS. 24 and 25 illustrate tank 212 and pair 208 and 210 of distributedRTDs shown in FIG. 16 and described in detail in connection therewith,but with distributed RTDs 208 and 210 disposed within a still well orstilling well 334 to mitigate the effects of fluid turbulence withintank 212. The primary problem relative to fluid tubulence relates toreadout inaccuracies which may occur because of turbulence in the liquidphase, but turbulence in the gas or air phase above the liquid may alsoadversely affect the readout, although normally to a lesser extent.Still well 334 serves to completely shield the two distributed RTDs 208and 210 from all such turbulence, and also serves to support distributedRTDs 208 and 210 along their vertical lengths.

Distributed RTDs 208 and 210 are normally calibrated in a still tank.The problem arises because distributed RTDs of the invention aresensitive not only to the presence of liquid, which disperses heat fromthe heated one of the distributed RTDs at a much greater rate than theair or gas disperses heat from the heated distributed RTD, but they arealso sensitive to fluid flow, which causes heat to be dispersed from theheated distributed RTD at an even greater rate than does still liquid.Thus, with distributed RTDs 208 and 210 calibrated in a still tank, andthen if the liquid should be circulating in the tank so as to cool theheated distributed RTD 208 or 210 more than when it was calibrated, thereadout would indicate a higher level of liquid than is actually in thetank, the circulation cooling augumenting the natural cooling caused bythe presence of the liquid. A typical example of turbulence which couldcause such a false readout would be swirling caused by filling oremptying the tank.

Still well 334 is supported vertically along a wall 214 of tank 212 bymeans of a series of vertically spaced brackets 336. The lower and upperends 338 and 340, respectively, of still well 334 are open, being spacedfrom respective bottom and top walls 216 and 218 of tank 212, so as topermit free vertical movement of liquid and air in still well 334 sothat the liquid level is still well 334 will follow the liquid level inthe rest of the tank, while nevertheless completely shielding the liquidand air which are in contact with distributed RTDs 208 and 210 from theeffects of turbulence.

The two distributed RTDs 208 and 210 are periodically supported alongthe length of still well 334 by a series of regularly vertically spacedsupport units generally designated 342, each of which consists of afirst pair of diametrically opposed support plates 344 and a second pairof diametrically opposed support plates 346 at right angles to plates344. One of the support units is shown in FIG. 25. Each of the supportplates 344 carries a pair of spaced support fingers 348, while each ofthe support plates 346 carries a single support finger 350. Thisarrangement provides three-fingered support for each of the distributedRTDs 208 and 210, with the fingers spaced substantially 120° apart abouteach distributed RTD 208 and 210. It will be seen that support fingers348 and 350 make essentially point contact with distributed RTDs 208 and210, such minimal contact area minimizing heat transfer away fromdistributed RTDs 208 and 210 which might otherwise disturb the accuracyof the readout. The side edges of support plates 344 and 346 arepreferably bowed inwardly as illustrated to maximize fluid flow areathrough the support unit 342.

Support plates 344 and 346 of each support unit 342 are preferably madeof stainless steel but it is to be understood that they may be made ofany suitable material.

While it is most convenient to support both of the distributed RTDs 208and 210 inside still well 334, it is to be understood that still well334 will accomplish its aforesaid function if only the heated one of thetwo distributed RTDs 208 or 210 is within still well 334.

FIGS. 26 and 27 illustrate duct elbow 234 of FIGS. 17 and 18 with itspair 230 and 232 of distributed RTDs disposed in a shroud generallydesignated 352 which is provided to increase the usable range orrangeability of distributed RTDs 230 and 232. If the mass flow rate ofthe fluid flowing through duct 234 is so high that further increases inthe mass flow rate will not cause substantial further cooling of heateddistributed RTD 230 or 232, then the signal output becomes saturated,which defines an upper flow sensing limit for the distributed RTDs.

Shroud 352 is provided with preferably round holes uniformly distributedalong its length and about its periphery so that only a fraction of thefull mass flow rate through duct 234 passes through the wall of shroud352 and thereby flows past distributed RTDs 230 and 232. These holes orperformations are designated 354, and are sized to keep the mass flowrate which is applied to distributed RTDs 230 and 232 well within theirsensing range despite the fact that the general fluid flow through duct234 might be outside the range.

As an extreme example of this mass flow rate range sensitivity, if waterwere passed through duct or conduit 234 without shroud 352, the signaloutput of distributed RTDs 230 and 232 might become saturated by a flowrate as low as only about one foot per second. By applying shroud 352around distributed RTDs 230 and 232, the sensing range could be broughtup to ten feet per second or more as desired by shroud 352 according tohow small the holes 354 are, with the same amount of cooling of heateddistributed RTD 230 or 232 as would have been caused by the one foot persecond flow rate without shroud 352.

Distributed RTDs 230 and 232 are preferably supported along the lengthof shroud 352 by a regularly spaced series of the support units 342 asseen in FIG. 27, such support units 342 having been described in detailhereinabove in connection with FIG. 25. Shroud 352 preferably enshroudsthe entire lengths of distributed RTDs 230 and 232 which extend acrossduct 234.

While it is most convenient to support both of the distributed RTDs 230and 232 inside shroud 352, it is to be understood that shroud 352 willaccomplish its aforesaid function if only the heated one of the twodistributed RTDs 230 or 232 is within shroud 352.

The flow of fluid through duct elbow 234 of FIGS. 17, 18 and 26 isgenerally symmetrical about the diametrical axis along which distributedRTDs 230 and 232 are deployed. However, if straight, linear distributedRTDs are employed, as the distributed RTDs 230 and 232 are shown in FIG.18, they would not provide a signal output that would De trulyrepresentative of the average mass flow rate, because thecross-sectional area of round duct 234 varies nonlinearly along thevertical axis of deployment shown in FIGS. 18 and 26, smallercross-sectional areas for the same incremental lengths of thedistributed RTDs being present near the upper and lower ends of thedistributed RTDs than near the centers of the distributed RTDs. FIG. 28illustrates a sinuous or serpentine configuration of distributed RTDswhich may be employed to compensate for this, providing equalincremental lengths of distributed RTDs for equal cross-sectional areasof flow sensed by these incremental lengths. To illustrate how thisworks, a series of horizontal lines 355 has been drawn across duct 356shown in FIG. 28 which divides the duct into substantially equalcross-sectional areas along the vertical axis of symmetry. The matchedpair of distributed RTDs which extends generally along the vertical axisof symmetry is designated 358, and includes the usual heated distributedRTD and unheated reference distributed RTD. The distributed RTD pair 358commences in generally straight end portions 360 proximate the upper andlower ends of the vertical axis of duct 356, and then curveshorizontally back and forth sinuously in loops 362 which swing wider andwider but in vertically tighter, closer together loops toward the centerof the vertical axis so that the same incremental length of distributedRTDs 358 is disposed in each of the equal areas defined between thehorizontal lines.

With this varying sinuous or serpentine configuration of lineardistributed RTDs 358, equal incremental lengths of linear distributedRTDs 358 will sense equal incremental areas across duct 356, to providea signal output truly representative of average mass flow rate where theflow is generally symmetrical about the axis of deployment ofdistributed RTDs 358. Thus, distributed RTDs 358 are configured so as torespond as a variable function that is proportional to duct area alongtheir lengths.

Where the flow of fluid is symmetrical about the center axis of a ductrather than a diametrical axis, a spiral configuration of lineardistributed RTDs of the invention may be employed as illustrated in FIG.29 to correlate equal incremental lengths of the distributed RTDs withequal incremental circular areas through which the incremental RTDlengths pass about the center axis of the duct, whereby the spiralconfiguration compensates for the fact that area increases in proportionto the square of the radius from the center axis. Thus, the spiralconfiguration provides a variable function to the linear distributedRTDs which corresponds to the variable square function of duct arearelative to radius.

Referring to FIG. 29, a first matched pair of distributed RTDs of theinvention, one of which is heated and the other of which is unheated toserve as a reference, is generally designated 366, and is arranged as apair of opposite spiral arms 368 and 370 disposed symmetrically aboutthe center axis of duct 364 and oriented generally along the verticaldiametrical axis as the duct is illustrated. Similarly, anotherparallel, matched pair of distributed RTDs of the invention, generallydesignated 372 and including a heated distributed RTD and an unheatedreference RTD, is arranged as a pair of opposite spiral arms 374 and 376symmetrically about the center axis of duct 364 and oriented generallyalong the horizontal diametrical axis as the duct is illustrated. Withthis arrangement, four separate sensing spiral arms are provided 90°apart, each of which has incremental lengths of the same length whichpass through respective circular incremental areas of equal area.Circular lines 378 drawn about the center axis of duct 364 define equalareas between them, including the central disk area enclosed within theinnermost circle 378, and it will be seen substantially equalincremental lengths of distributed RTDs 366 are disposed betweensuccessive circular lines 378.

The array of four uniformly separated spiral arms 90° apart provides anexcellent overlapping matrix of spiral arms to sense the mass flow ratein all sectors over the area of duct 364, to provide an excellentaverage readout of the mass flow rate despite the fact that there may beconsiderable variations in the flow rate for different regions in theoverall cross-sectional area of duct 364. If even greater averaging isdesired, additional spirals may be provided, as for example eight spiralarms provided by four matched pairs of linear distributed RTDs, theeight spiral arms being 45° apart. Alternatively, if the flow isrelatively uniform over the cross-sectional area of duct 364, then twospiral arms 180° apart such as the two spiral arms 368 and 370 of lineardistributed RTDs 366 will suffice. Typically, the flow in a duct may besymmetrical about one axis, as for example the vertical axis in FIG. 29,in which case the generally averaged distribution of spiral arms 368 and370 about the vertical axis will give a good average readout. As afurther alternative, if desired, the linear RTDs may be arranged in buta single spiral arm such as spiral arm 368.

The distributed RTDs of the invention have the characteristic ofrelatively higher sensitivity to relatively lower flow rates. Forexample, the distributed RTDs of FIG. 29 would have a higher sensitivitywithin a range of flow rates from zero to five feet per second thanwithin a range of flow rates from 20 to 25 feet per second. If thehighest possible sensitivity to flow rate change is desired, and theempircal mass flow rate is not as important, then it is desirable tofavor distribution of the distributed RTD material in the zone of lowerflow rate where there are differing flow rates in a duct. Looking atFIG. 29 for example, the flow rate radially outwardly near the wall ofduct 364 will be less than the flow rate radially inwardly near thecenter of the duct. Thus, to take advantage of this greater sensitivityachievable at slower flow rates, spiral arms 368, 370, 374 and 376 wouldbe curved to be more tangentially related to the wall of duct 364 so asto place a larger amount of RTD material in this zone of slower fluidflow. Simiarly, matrices of distributed RTDs of the invention other thanthe spiral matrix shown in FIG. 29 may be arranged to have aproportionately greater amount of RTD material in zones of lesser flowrates wherever they may be over the area of the duct to take advantageof the increased sensitivity obtainable from the lesser flow rates.

If, on the other hand, mass flow rate averaging were more important thansuch added sensitivity, then to compensate for the lesser flow rateproximate the wall of duct 364, the spiral arms may be bent moreradially outwardly proximate their outer ends to provide less RTDmaterial, and hence less sensitivity, in such region of lesser flowrate. In this case, the ends of the spiral arms would be less tangentialto the wall of duct 364. Similarly, matrices of distributed RTDs of theinvention other than the spiral matrix shown in FIG. 29 may be arrangedto have proportionately lesser RTD material in zones of lesser flowrates wherever they may be over the area of the duct.

FIG. 30 illustrates a form of distributed RTD of the invention whichemploys a continuous outer sheath that houses not only the RTD andheater filaments of the sensing portion of the invention, but alsohouses the electrical conductors which electrically connect the RTD andheater filaments of the sensor to remote detection circuitry so thatremote readout can be obtained with no interruption of the sheath, andwithout need for any electrical connectors, junction boxes or the like.This form of the invention has particular utility in atomic power plantswhere it is undesirable to have electrical junctions inside the reactorvessel or inside the containment vessel, junctions being by far the mostlikely points of failure, and being difficult to access in some regionsof the power plant. The continous sheath RTD and conductor device ofFIG. 30 positively precludes any such failure.

Referring to FIG. 30, the distributed RTD/conductor device is generallydesignated 380, and for convenience is disclosed in a simplified formhaving a single RTD filament and a single heater filament. The primarystructural basis is outer sheath 382, preferably of metal such asstainless steel. Extending coextensively with the length of outer sheath382 is insulation body 384, preferably of mineral insulation material.Parallel RTD and heater filaments 386 and 388, respectively, extend thelength of the sensor portion of distributed RTD/conductor sheath 382 andinsulation body 384, which ends at the transverse phantom line 390. At390, the ends of filaments 386 and 388 are connected, preferably bywelding, to ends of respective electrical conductors 392 and 394 made ofhighly conductive material such as copper. Sheath 382 and insulationbody 384 continue uninterruptedly from the sensor to carry theelectrical conductors 392 and 394 to the instrumentation where thereadout is developed, no matter how remote such instrumentation may be,thus avoiding any electrical connectors, junction boxes, or the like atany location in the plant except where the instrumentation is locatedand monitored. Thus, sheath 382 and insulation body 384 may beconsidered as having distributed RTD portions which house the filaments386 and 388, and conductor portions which house the conductors 392 and394, with the conductor portions being continuous extensions of thedistributed RTD portions.

Distributed RTD/conductor device 380 is manufactured in the same manneras the other forms of the invention, starting with a relatively shortand thick billet-like assembly which is swaged and/or drawn out to thefinished long, thin device.

While the present invention has been described with regard to particularembodiments, modifications may readily be made by those skilled in theart, and it is intended that the claims cover any such modificationswhich fall within the scope and spirit of the invention as set forth inthe appended claims.

I claim:
 1. A method of measuring average mass flow velocity of gas in aduct where the gas has a nonuniform flow velocity distribution acrossthe duct, the method comprising the steps of:deploying a physically andelectrically matched pair of elongated, relatively stiff, heater-type,wire-like distributed RTD sensors in an array across said duct from oneside to the other, said RTD sensors having heater means, the heatermeans of each RTD sensor being co-extensive in length with the RTD;heating only a selected one of said matched pair of distributed RTDsensors so that the other one serves as a thermal reference with respectto the heated RTD sensor; thermally sensing by means of the RTD sensorsthe motion of the gas through the duct and developing signal outputs ofthe RTD sensors representative of that gas motion; transmitting thesignal outputs to a location remote from the RTD sensors; and detectingand comparing the signal outputs of said pair of distributed RTD sensorsto measure the average mass flow velocity of the gas.
 2. The method ofclaim 1, which comprises providing said distributed RTD sensing meanswith substantially linear function sensitivity along its length so thatsaid signal output will substantially linearly represent the location ofsaid interface.
 3. The method recited in claim 1, and comprising thefurther step of providing said distributed RTD sensing means withsubstantially linear sensitivity along its length so that said signaloutput will substantially linearly represent the average mass flow rateof fluid flowing past said distributed RTD sensing means.
 4. The methodrecited in claim 1, and comprising the further step of deploying saiddistributed RTD sensor across said duct in a nonlinear configurationwhich places substantially equal lengths of said distributed RTD sensorsacross substantially equal cross-sectional flow areas of said duct sothat said signal output will substantially linearly represent averagemass flow rate of fluid through said duct.
 5. A method of measuringaverage mass flow velocity of gas in a duct where the gas has anonuniform flow velocity distribution, the method comprising the stepsof:deploying at least one elongated, relatively stiff, wire-likedistributed RTD sensing means across said duct, the RTD sensing meanshaving heater means co-extensive in length with the RTD sensing means,said distributed RTD sensing means being deployed in said duct in anonlinear configuration which places relatively greater lengths of saiddistributed RTD sensing means across zones of relatively slower fluidflow rates in said duct so as to provide an overall increase insensitivity of said distributed RTD sensing means to the mass flow rateof fluid through said duct, thermally sensing, by the RTD sensing means,physical information about the mass flow rate of the gas external to theRTD sensing means and developing signal outputs of the RTD sensing meansrepresentative of said information; transmitting the signal outputs to aremote location; and detecting and interpreting the signals at theremote location to obtain the desired information about the mass flowvelocity of fluid through said duct from the RTD signal outputs.
 6. Amethod of measuring average mass flow velocity of gas in a duct wherethe gas has a nonuniform flow velocity distribution, the methodcomprising the steps of:deploying at least one elongated, relativelystiff, wire-like distributed RTD sensing means across said duct, the RTDsensing means having heater means co-extensive in length with the RTDsensing means, said distributed RTD sensing means being deployed in saidduct in a nonlinear configuration which places relatively lesser lengthsof said distributed RTD sensing means across zones of relatively slowerfluid flow rates in said duct so as to provide increased accuracy insensing the average mass flow rate of fluid through said duct, thermallysensing, by the RTD sensing means, physical information about the massflow rate of the gas external to the RTD sensing means and developingsignal outputs of the RTD sensing means representative of saidinformation; transmitting the signal outputs to a remote location; anddetecting and interpreting the signals at the remote location to obtainthe desired information about the mass flow velocity of fluid throughsaid duct from the RTD signal outputs.